Interaction between like-charged polyelectrolyte-colloid complexes in electrolyte solutions: a Monte Carlo simulation study in the Debye-H'uckel approximation

摘要

We study the effective interaction between differently chargedpolyelectrolyte-colloid complexes in electrolyte solutions via Monte Carlosimulations. These complexes are formed when short and flexible polyelectrolytechains adsorb onto oppositely charged colloidal spheres, dispersed in anelectrolyte solution. In our simulations the bending energy between adjacentmonomers is small compared to the electrostatic energy, and the chains, onceadsorbed, do not exchange with the solution, although they rearrange on theparticles surface to accomodate further adsorbing chains or due to theelectrostatic interaction with neighbor complexes. Rather unexpectedly, whentwo interacting particles approach each others, the rearrangement of thesurface charge distribution invariably produces anti-parallel dipolar doublets,that invert their orientation at the isoelectric point. These findings clearlyrule out a contribution of dipole-dipole interactions to the observedattractive interaction between the complexes, pointing out that suchsuspensions can not be considered dipolar fluids. On varying the ionic strengthof the electrolyte, we find that a screening length, short compared with thesize of the colloidal particles, is required in order to observe the attractionbetween like charged complexes due to the non-uniform distribution of theelectric charge on their surface ('patch attraction'). On the other hand, bychanging the polyelectrolyte/particle charge ratio, the interaction betweenlike-charged polyelectrolyte-decorated (pd) particles, at short separations,evolves from purely repulsive to strongly attractive. Hence, the effectiveinteraction between the complexes is characterized by a potential barrier,whose height depends on the net charge and on the non-uniformity of theirsurface charge distribution.